SharePoint Designer 2010: You do not have permission to do

Question

Hi,

 

My account have site collection administration privilages and i'm able to open the site in Sharepoint designer 2010 but when i'm trying to open the list forms (default / custom forms) i am getting the following error

 

"You do not have permission to do this operation. Ask your website administrator to change your permissions and then try again, or log on with a user account that has this permission. To log on with a different user account, click OK".

 

How can i resolve this error? I tried different thread from internet and couldnot able to resolve the issue. What could be issue though my account have full control permissions? and I'm Not facing any issue when login with service account